It is known to provide a vehicle with a transmission control system to control the operation of a continuously variable transmission (CVT) by varying its transmission ratio in a continuous manner. An increase in transmission ratio causes a decrease in road speed for a given engine speed and a decrease in transmission ratio causes an increase in road speed for a given engine speed.
It is a known advantage of equipping vehicles with a CVT that, by operating the prime mover at its optimum efficiency and varying the transmission ratio in a continuous manner, it is possible to achieve optimum acceleration. It is, however, a known disadvantage of such systems that on over-run, because the transmission ratio is varied in a continuous manner, there is very little engine braking to assist deceleration.
In U.S. Pat. No. 5,527,231 a system is disclosed which detects an over-run condition and provides for a continuous increase in transmission ratio while the brakes are being operated, so as to provide engine braking on over-run to reduce the stress on the service brakes.
In U.S. Pat. No. 5,527,232 a system is disclosed which detects an over-run condition and fixes the transmission ratio at the value it possessed on commencement of over-run, for example, when the accelerator pedal is released rapidly on entry to a bend. This system prevents a decrease in transmission ratio under such circumstances, which would provide little engine braking.